This invention relates to organic photoreceptors suitable for use in electrophotography and, in particular, in liquid electrophotography.
In electrophotography, a photoreceptor in the form of a plate, belt, or drum having an electrically insulating photoconductive element on an electrically conductive substrate is imaged by first uniformly electrostatically charging the surface of the photoconductive layer, and then exposing the charged surface to a pattern of light. The light exposure selectively dissipates the charge in the illuminated areas, thereby forming a pattern of charged and uncharged areas. A liquid or solid toner is then deposited in either the charged or uncharged areas to create a toned image on the surface of the photoconductive layer. The resulting visible toner image can be transferred to a suitable receiving surface such as paper. The imaging process can be repeated many times.
Both single layer and multilayer photoconductive elements have been used. In the single layer embodiment, a charge transport material and charge generating material are combined with a polymeric binder and then deposited on the electrically conductive substrate. In the multilayer embodiment, the charge transport material and charge generating material are in the form of separate layers, each of which can optionally be combined with a polymeric binder, deposited on the electrically conductive substrate. Two arrangements are possible. In one arrangement (the "dual layer" arrangement), the charge generating layer is deposited on the electrically conductive substrate and the charge transport layer is deposited on top of the charge generating layer. In an alternate arrangement (the "inverted dual layer" arrangement), the order of the charge transport layer and charge generating layer is reversed.
In both the single and multilayer photoconductive elements, the purpose of the charge generating material is to generate charge carriers (i.e., electron-hole pairs) upon exposure to light. The purpose of the charge transport material is to accept one of these charge carriers and transport them through the charge transport layer in order to discharge a surface charge on the photoconductive element.
To produce high quality images, particularly after multiple cycles, it is desirable for the charge transport material to form a homogeneous solution with the polymeric binder and remain in solution. In addition, it is desirable to maximize the amount of charge which the charge transport material can accept (indicated by a parameter known as the acceptance voltage or "V.sub.acc "), and to minimize retention of that charge upon discharge (indicated by a parameter known as the residual voltage or "V.sub.res ").
Liquid toners generally produce superior images compared to dry toners. However, liquid toners also can facilitate stress crazing in the photoconductive element. Stress crazing, in turn, leads to printing defects such as increased background. It also degrades the photoreceptor, thereby shortening its useful lifetime. The problem is particularly acute when the photoreceptor is in the form of a flexible belt included in a compact imaging machine that employs small diameter support rollers (e.g., having diameters no greater than about 40 mm) confined within a small space. Such an arrangement places significant mechanical stress on the photoreceptor, and can lead to degradation and low quality images.
One solution developed has been to provide a barrier layer to an organic photoreceptor. Conventional barrier layers have been formed from a variety of materials, examples of which include crosslinkable siloxanol-colloidal silica hybrids (as disclosed, e.g., in U.S. Pat. Nos. 4,439,509; 4,606,934; 4,595,602; and 4,923,775); a coating formed from a dispersion of hydroxylated silsesquioxane and colloidal silica in an alcohol medium (as disclosed, e.g., in U.S. Pat. No. 4,565,760); a polymer resulting from a mixture of polyvinyl alcohol with methyl vinyl ether/maleic anhydride copolymer (e.g., in U.S. Pat. No. 5,124,220); a coating formed from an organic polymer (such as polyacrylates, polyurethanes, polyvinyl acetals, sulfonated polyesters, and mixtures of polyvinyl alcohol with methylvinylether/maleic anhydride copolymer) and silica (e.g., in International Publication No. WO 95/02853); and polyvinyl butyral crosslinked with methylvinylether/maleic anhydride copolymer, such as that commercially available under the trade designation GANTREZ AN 169, from ISP, Wayne, N.J.
However, it has been found that these barrier layers do not provide an organic photoconductive layer with adequate protection from liquid contact when a liquid toner is utilized. Furthermore, it has been found that these barrier layers do not possess sufficient resiliency to be used in belt form, as evidenced by stress fractures that developed in the charge transport layers when subjected to stress testing in the presence of a liquid toner.